Unlike an automobile that uses an air-to-coolant heat exchanger (i.e. a radiator) to maintain proper operating temperatures, marine engines use sea water to either directly cool the engine (raw water cooling) or a sea water-to-coolant heat exchanger (closed water cooling). Both systems require a constant flow of sea water to prevent overheating and ultimately engine damage.
The options for collecting engine cooling water for offshore performance boats are limited to three methods. The most common is to use the built-in water passages that are available on most stern drives from the factory. This is adequate for most applications since the water pickups are, by design, are placed in an area that has a good flow but lower pressures such as on the contour of a housing or on the backside of a gear case.
In applications where stern drive intakes are not available, such as surface drives or shaft drives, then either a through hull pickup or externally mounted water pickups are the only option. A through hull water pickup is by far the best option but determining the exact location in the hull of a step bottom boat is challenging and typically only installed at the factory on very high speed catamarans.
The result is that the most common, non-stern drive mounted, water pickup is externally fastened to the transom at the bottom edge of the hull. Most are mounted in a fixed position but some have adjustable brackets that allow the pickup tube to be moved up and down in relation to the bottom of the hull. One would think that one can solve the problem by mounting the water pickup well below the surface of the water. However, this is not the case, particularly at the speeds many performance boats are capable of these days. A poorly designed or mounted water pickup can generate extreme water pressures in the engine's cooling system, at even modest speeds. The dynamic pressure generated by a fully submerged water pickup moving at 80 MPH is 93 PSI, which is more than four times the required pressure. Such extreme water pressure can lead to burst hoses and can sink a boat in a very short period of time. In raw water cooled engines the extreme pressure can damage head gaskets and literally fill the engine block with water in a matter of seconds. A raw water cooled engine typically requires less than 20 PSI so clearly one cannot simply mount them below the bottom of the boat and maintain safe water pressures.
The boat's trim and the water conditions also play a factor and can lead to “dead spots” where the water pressure is inadequate for the speed and trim angle at which the boat is being operated. For example, running in moderate seas, the operator will likely reduce speed and trim the boat down to have the bow break the waves sooner for a smoother ride. A pickup that is positioned for proper pressures a high speeds will likely experience low water pressure under these bow down conditions, forcing the operator to comprise the ride quality for water pressure.
External water pickup placement that is optimized for flow will most likely reduce boat speed due to the increased hydrodynamic drag from the pickup being placed below the bottom of the hull. At high speeds this condition can lead to excessive pressure. Water pickup placement that is optimized for operating at high speeds almost certainly leads to poor water flow at midrange and cruising speeds that can melt the rubber exhaust hoses and release hot exhaust into the engine compartment or cause the engine damage due to overheating.
There are adjustable water pickups on the market but these require that the boat be removed from the water to make the adjustment and they are still a compromise between too much and too little water pressure. Some race boats have designed water pickups that can be adjusted via cable and lever in the cockpit. However, these require constant monitoring of the water pressure and adjustment of the pickup. Making the manual adjustments is a little easier to do in a race boat that always has two drivers onboard.
The most popular way of managing the water pressure is to install a pressure regulator on the water line leading from the pickup to the engine. When the pressure exceeds a set value the valve opens and allows the excess water to be routed overboard. This solution requires that the water pickup be mounted well below the bottom of the boat to ensure good water flow under all operating conditions. However, these valves do not address the issue of the increased drag caused by the extended water pickup. Another disadvantage is that they require extra plumbing and a thru-hull fitting for the water dump.
As can be seen, the prior art has not solved the problem of providing a simple and automatic solution for providing an automatic water pickup that adjusts for pressure. Manually adjusting the pickup height will solve the problem, but this isn't a practical solution in a recreation boat. The pressure regulator will dump the excess pressure but does not reduce the drag caused by a water pickup that is mounted far enough below the hull as to produce adequate water pressure under all conditions. It is believed that this drag can reduce the boat speed by at least three to five MPH. In most boats one would have to increase engine output by 50-100 HP to produce a five 5 MPH gain in speed.